This invention relates to a circuit and method for measuring pulsating torque in ac motors operating from a current source inverter, and more particularly to a scheme for the on-line computation of the instantaneous pulsating component of electromagnetic torque without the need for search or flux sensing coils in the machine.
Many applications including traction drive systems require the precise regulation of motor torque. The development of current source or controlled current inverters, which supply rectangular non-sinusoidal currents to the motor windings, has resulted in efforts to apply this device to adjustable speed ac induction motor drives. One of the weaknesses of present control strategies is that the torque pulsations due to the harmonic or cogging component of electromagnetic torque can be severe at very low machine frequencies and result in instabilities and uneven running. For a six-pulse, polyphase full wave bridge inverter, torque ripple occurs because of the presence of the sixth, twelfth, and eighteenth harmonic components in the non-sinusoidal motor current in addition to the fundamental motor frequency. The torque pulsations are especially troublesome upon starting up or when passing through zero speed to reverse the direction of rotation, and can be eliminated by modulating the input dc current fed to the inverter.
In practice, motor parameters vary with temperature and frequency so that actual real-time measurement of the pulsating torque and closed-loop feedback control is necessary for the precise regulation of torque rather than relying on open loop compensation. A closed loop technique for reducing torque ripple which requires the continuous calculation of torque from the sensed motor voltage and current is disclosed in U.S. Pat. No. 3,919,609 to Klautschek et al., however the torque calculated in this case is the total or shaft plus pulsating torque. Other prior art techniques for calculating torque use flux coils embedded in the stator slots of the machine. The present invention realizes means for calculating only the instantaneous pulsating component of electromagnetic torque, which is exact and independent of changes in motor parameters and is implemented by the sensing of readily available currents and motor terminal voltages. Utilization of the change of instantaneous torque signal as a decogging feedback control variable in a controlled current inverter motor drive is disclosed and claimed in the concurrently filed application, Ser. No. 817,626 by the inventor, which is assigned to the same assignee as this invention.